The present invention relates generally to methods for treating cupola and electric arc furnace dust and other waste materials generated during manufacturing processes. More specifically, the present invention relates to an environmentally acceptable closed system process for treating environmentally hazardous heavy metal oxides generated during ferrous metal and nonferrous scrap remelt operations.
Foundries use a range of melt technologies to produce liquid iron and steel for casting. The melt technologies used include cupolas, induction furnaces, and electric arc furnaces. Scrap metal is fed into the furnace or melt vessel and the metal is subjected to intense heat to induce melting. As a byproduct of that melting process, waste gases and particulates are generated and collected above the melt vessel.
Federal and state or provincial environmental regulations in the United States, Canada, and other countries require the collection of these solid and gaseous wastes as well as the removal of the particulate wastes before the exhaust is released to the atmosphere. (This application principally discusses United States Environmental Protection Agency (“EPA”) regulations, but comparable regulations are applicable in individual states and other countries.) The device used to capture these wastes is any one of a series of filtering devices, most commonly called a bag house. The solid wastes collected by these filters are typically disposed in a landfill, either as a non-hazardous waste (US EPA Subtitle D wastes) or as a hazardous waste (US EPA Subtitle C wastes).
Disposal of hazardous waste often requires one or more types of waste treatment before disposal, and the management of these hazardous wastes is typically more expensive than non-hazardous wastes. Using EPA-developed extraction testing procedures, foundry bag house wastes that are hazardous usually have unacceptable levels of one or more RCRA-regulated heavy metals, including arsenic, barium, cadmium, chromium, lead, mercury, selenium and silver. Once these filter wastes are deemed hazardous, the US EPA requires that they be treated until the toxic metals in the waste meet a universal treatment standard (UTS) set for each hazardous heavy metal contaminant.
Foundries and other scrap remelt operations are regulated under the Solid Waste Disposal Act, as amended by the Resource Conservation and Recovery Act of 1976 and the Hazardous and Solid Waste amendments of 1984 (collectively “RCRA”). RCRA controls and regulates the collection of bag house dust and like hazardous wastes. If a system generates hazardous waste, then treatment of the hazardous waste requires a RCRA treatment permit prior to treatment of the waste for disposal. See, 40 CFR Section 261.4 (c).
The process of securing a RCRA permit can take up to one year and the applicant may incur a cost of up to $100,000.00. Therefore, if a more efficient process for treating metallic oxide impregnated dust within the manufacturing processing unit is used, treating the dust before it is classified as “waste” for regulatory purposes, a RCRA permit is not needed. This will result in a savings in of both time and money to the foundry operator.
Foundries purchase scrap iron and steel from a wide array of sources, and it is not uncommon for the scrap iron or steel to be somewhat contaminated by “tramp” RCRA 8 heavy metals. These heavy metals can come from any number of sources, including car batteries (lead), wheel balancing weights (lead), brass plumbing fixtures (lead) and plated metal surfaces (cadmium and chromium). When the iron and steel are melted, these tramp heavy metals also melt and then vaporize, since their vaporization point is lower than the melting point of iron and steel. The heavy metal gases leave the melting chamber with the exhausted gas and particulates, and as they cool, the heavy metals re-form as metal oxide particulate and end up in the bag house dust collected from the melt process. Since these oxides tend to be toxic and environmentally mobile, federal and state regulatory authorities want the wastes segregated and treated before disposal.
Typically, the bag house dust contains a variety of types of metallic oxide particles including RCRA 8 toxic heavy metals in sufficiently high concentrations to be classified as hazardous by the US Environmental Protection Agency and its state counterparts. Because this bag house dust is generated by subjecting scrap metal to high temperatures, the dust contains oxides and is extremely dry.
Typical foundry furnace bag house dusts can include oxides of the following component metals:
ComponentWeight PercentIron15-18percentManganese2percentNickelless than 1percentLead2-6percentCadmium1-2percentMagnesium1-4percentChromium1percentZinc35-40percentOtherBalance
Because this dust contains hazardous levels of regulated metallic oxides, it is necessary to treat the dust before disposal. To this end, prior art foundry processes have, for example, treated the dust by feeding the hazardous dust from the bag house to a feed silo that mixes the hazardous dust with an additive so that the dust can then be disposed of.
Over the past twenty years, several parties found that these wastes could be rendered non-hazardous by injecting chemicals into the furnace or into the exhaust ducts of the furnace. These chemicals commingled with the wastes during air transport, producing a non-hazardous waste in the bag house. After careful review, the US EPA and the state counterparts to the US EPA determined that this was an allowable part of the manufacturing process that would be exempt from the requirement that permits must be obtained for all hazardous waste treatment processes before use at a generator's site. These processes included the injection of very fine calcium silicates combined with calcium sulfoaluminate, as well as the injection of various types of phosphates combined with magnesium oxide and/or calcium oxide (lime)—bearing compounds. These processes are disclosed in U.S. Pat. Nos. 4,878,944, 5,037,479, 6,857,998, U.S. Ser. No. 06/920,922, and Canadian Patent No. 1279876, each incorporated here by reference.
In the case of the finely ground calcium silicates combined with calcium sulfoaluminate, researchers found that injection of chemicals at rates of 15 to 20% by weight was effective in producing non-hazardous wastes under a wide range of regulated (RCRA 8) heavy metal contamination levels. This was and is an alternative method historically used at multiple foundry facilities over almost a two decade period of time. As an example, many of those foundries had lead and cadmium levels in the bag house dust that typically ranged from 2,700 ppm to 10,400 ppm for lead and 100 to 300 ppm for cadmium. This waste and a 15% to 20% by weight addition rate of blends of calcium silicates and calcium sulfoaluminates produced waste materials compliant with US EPA Subtitle D non-hazardous waste management standards. In the case of the phosphate admixtures, independent data suggests that addition rates of approximately 3% to 20% by weight were also successful in stabilizing RCRA 8 heavy metals and rendering wastes non-hazardous.